Polylactic Acid (PLA) as a Biodegradable Polymer
Advanced technology in petrochemical based polymers has brought many benefits to mankind (Ray, S. S., Yamada, K., Okamoto, M., Ueda, K., Control of biodegradability of polylactide via nanocomposite technology. Macromolecular Materials and Engineering, 2003. 288(3): p. 203-208). However, it has become evident that the ecosystem is disturbed as a result of the use of non-degradable plastic materials for disposable items. The environmental impact of persistent plastic wastes is a growing global concern, and alternative disposal methods are limited. Since the petroleum resources are finite there is an urgent need to develop renewable source based environmentally benign polymeric materials, especially in short-term packaging and disposable applications, which do not involve the use of toxic or noxious components in their manufacture, and can be composted to biodegradable products.
Polylactic acid (PLA) is a highly versatile, biodegradable, aliphatic polyester that can be derived from 100% renewable resources, such as corn and sugar beets. Cargill Dow LLC has developed a patented, low-cost continuous process for the production of lactic acid-based polymers (Drumright, R. E., P. R. Gruber, and D. E. Henton, Polylactic acid technology. Advanced Materials, 2000. 12(23): p. 1841-1846). This low-cost biodegradable PLA increases the range of controlled-release options for agricultural utility.
PLA is well-known as a biodegradable polymer in nature or compost. However, PLA is not easily biodegraded by microorganisms or enzymes in soil or water in nature, unless the period exceeds two years. This degradation rate is not sufficiently fast for applications where degradation times of a few months are desired (Funabashi, M. and M. Kunioka, Biodegradable composites of poly(lactic acid) with cellulose fibers polymerized by aluminum triflate. Macromolecular Symposia, 2005. 224: p. 309-321).
The environmental degradation of PLA occurs by a two-step process. During the initial phase of degradation, the high molecular weight polyester chains hydrolyze to lower molecular weight oligomers. This reaction can be accelerated by acids or bases and is affected by both temperature and moisture levels. Embrittlement of the plastic occurs during this step at a point where the molecular number (Mn) decreases to less than about 40000. At about this same Mn, microorganisms in the environment continue the degradation process by converting these lower molecular weight components to carbon dioxide, water, and humus. The structural integrity of molded PLA particles decreases as the molecular weight drops and eventually the particle disintegrates. A microstructure or morphology related to the hydrophilicity of PLA is very important for controlling the hydrodegradation rate. Funabashi et al. (Masahiro Funabashi, M. K., Biodegradable Composites of Poly(lactic acid) with Cellulose Fibers Polymerized by Aluminum Triflate. Macromolecular Symposia, 2005. 224(1): p. 309-321) reported that the biodegradability of PLA composites was accelerated by the existence of paper fibers or cotton fibers.
PLA is of increasing commercial interest since it can be made from completely renewable agricultural products, has comparable properties to many petroleum-based plastics and is readily biodegradable (Drumright, R. E., P. R. Gruber, and D. E. Henton, Polylactic acid technology. Advanced Materials, 2000. 12(23): p. 1841-1846; Tsuji, H. and Y. Ikada, Blends of aliphatic polyesters.2. Hydrolysis of solution-cast blends from poly(L-lactide) and poly(epsilon-caprolactone) in phosphate-buffered solution. Journal of Applied Polymer Science, 1998. 67(3): p. 405-415). High molecular weight PLA is generally produced by the ring opening polymerization of lactide monomer, which in turn is obtained from the fermentation of sugar feed stocks, corn, etc. (Lunt, J., Large-scale production, properties and commercial applications of polylactic acid polymers. Polymer Degradation and Stability, 1998. 59(1-3): p. 145-1524). Even when burned PLA produces no nitrogen oxide gases, only one-third of the combustible heat generated by polyolefins, and does not damage the incinerator thus, providing a significant energy savings (Ray, S. S., Yamada, K., Okamoto, M., Ueda, K., Control of biodegradability of polylactide via nanocomposite technology. Macromolecular Materials and Engineering, 2003. 288(3): p. 203-208).
PLA has been widely used in various biomedical applications due to its biodegradability, biocompatibility, good mechanical properties and solubility in common solvents for processing (Kim, K., Yu, M., Zong, X. H., Chiu, J., Fang, D. F., Seo, Y. S., Hsiao, B. S., Chu, B., Hadjiargyrou, M., Control of degradation rate and hydrophilicity in electrospun non-woven poly(D,L-lactide) nanofiber scaffolds for biomedical applications. Biomaterials, 2003. 24(27): p. 4977-4985). However, PLA has a slow biodegradation rate even in the non-crystalline form of poly(D,L-lactide) as well as in enantiomeric semicrystalline forms of poly(D-lactide) and poly(L-lactide). Thus degradation of PLA-based materials may take too long for many biomedical applications.
The degradation of aliphatic polyester is based on a hydrolytic reaction (Kim, K., Yu, M., Zong, X. H., Chiu, J., Fang, D. F., Seo, Y. S., Hsiao, B. S., Chu, B., Hadjiargyrou, M., Control of degradation rate and hydrophilicity in electrospun non-woven poly(D,L-lactide) nanofiber scaffolds for biomedical applications. Biomaterials, 2003. 24(27): p. 4977-4985). When water molecules attack ester bonds in the polymer chains, the average length of the degraded chains decreases. Eventually, the process results in short fragments of chains with carboxyl end groups that become soluble in water. Very often, the molecular weights of some fragments are still relatively large so that the corresponding diffusion rates are slow. As a result, the remaining oligomers will lower the local pH value, catalyze the hydrolysis of other ester bonds and speed up the degradation process. This mechanism is termed autocatalysis, and is frequently observed in thick biodegradable implants. However, if the dimension of the implant is small and the diffusion path is short, the hydrophilic oligomers can quickly escape from the surface. This is the case with electrospun scaffolds, in which the dimension of the nanofibers is small and the diffusion length for the degraded byproducts (hydrophilic oligomers) is short. As a result, the possibility of autocatalysis in electrospun scaffolds is very limited (Kim, K., Yu, M., Zong, X. H., Chiu, J., Fang, D. F., Seo, Y. S., Hsiao, B. S., Chu, B., Hadjiargyrou, M., Control of degradation rate and hydrophilicity in electrospun non-woven poly(D,L-lactide) nanofiber scaffolds for biomedical applications. Biomaterials, 2003. 24(27): p. 4977-4985).
The degradation of PLA is primarily due to hydrolysis of the ester linkages, which occurs more or less randomly along the backbone of the polymer. Hydrolysis requires the presence of water according to the following reaction: (David E. Henton, P. G., Jim Lunt, and Jed Randall, Polylactic Acid Technology, in Natural Fibers, Biopolymers, and Biocomposites, M. M. Amar K. Mohanty, and Lawrence T. Drzal, Editor. 2005, CRC Press. p. 528-569).

The rate of hydrolysis is determined by its intrinsic rate constant, water concentration, acid or base catalysis, temperature, and morphology (David E. Henton, P. G., Jim Lunt, and Jed Randall, Polylactic Acid Technology, in Natural Fibers, Biopolymers, and Biocomposites, M. M. Amar K. Mohanty, and Lawrence T. Drzal, Editor. 2005, CRC Press. p. 528-569). Two major challenges to the stabilization of PLA with regard to hydrolysis are the fact that it is quite permeable in water and that the hydrolysis reaction is autocatalytic. The autocatalytic hydrolysis reaction is as follows:
The following equation describes the decrease in ester concentration [E] over time:
                                          d            ⁡                          [              E              ]                                            d            ⁢                                                  ⁢            t                          =                                            k              ⁡                              [                                                      -                    C                                    ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  H                                ]                                      ⁡                          [                                                H                  2                                ⁢                O                            ]                                =                                    d              ⁡                              (                                  1                  /                  Mn                                )                                                    d              ⁢                                                          ⁢              t                                                          (        1        )            For a random chain scission, [—COOH]∝1/Mn and the product [H2O][E] is constant. Rearranging toMnd(1/Mn)=kdt  (2)the integrated form becomesLn Mn,t=Ln Mn,0−kt  (3)where Mn,t=number average molecular weight at time t, Mn,0=number average molecular weight at time zero, and k is the hydrolysis rate constant. The kinetics were derived by Pitt et al. (Pitt, C. G., Jeffcoat, A. R., Zweidinger, R. A., Schindler, A., Sustained Drug Delivery Systems.1. Permeability of Poly(Epsilon-Caprolactone), Poly(Dl-Lactic Acid), and Their Copolymers. Journal of Biomedical Materials Research, 1979. 13(3): p. 497-507), and were again supported by Tsuji (Tsuji, H., Polylactides. 4 ed. Biopolymer, ed. A. Steinbüchel. 2001, Weinheim: Chichester: Wiley-VCH. 129-177).
The morphology of PLA (i.e., size and shape) plays an important role in its hydrolytic degradation (Li, S., Hydrolytic Degradation Characteristics of Aliphatic Polyesters Derived from Lactic and Glycolic Acids. Journal of Biomedical Materials Research Part B: Applied Biomaterials, 1999. 48(3): p. 342-353). If the size is very small as in the case of micro-particles, slim fibers, or thin films, the degradation should be slower than for large-sized materials because in the former case, no autocatalytic degradation occurs because of the easier diffusion of oligomers and the neutralization of carboxyl end groups (Li, S., Hydrolytic Degradation Characteristics of Aliphatic Polyesters Derived from Lactic and Glycolic Acids. Journal of Biomedical Materials Research Part B: Applied Biomaterials, 1999. 48(3): p. 342-353).
Controlled Release of Pesticides
The coating of pesticides using a polymer, a form of microencapsulation technology, has significant commercial value for improved resistance of pesticides to photolytic degradation and runoff. The coating can be a single polymer film completely entrapping the pesticide (such as single-core microcapsule) or it can be blended with the pesticide such that the pesticide is homogeneously distributed as a distinct second phase throughout the polymer continuous phase (commonly described as microsphere or multicore particle). If the polymer and pesticide are miscible and solidification occurs above ambient temperature, then films can be prepared.
Microencapsulation is not widely used because of the perceived hurdles associated with the cost. Low-cost biodegradable polymers with possible agricultural utility have been developed and commercialized. The range of physical properties of the polymers and their varied degradation rates permit the selection of a polymer to match a customized release rate.
There is therefore a need in the art for biodegradable polymers that hydrolyze quickly in the environment. There is also a need for polymer coatings for controlled release of pesticides into the environment for agricultural applications.
Citation or identification of any reference in Section 2, or in any other section of this application, shall not be considered an admission that such reference is available as prior art to the present invention.